The present invention generally relates to mass flowmeters, and more particularly to a mass flowmeter which can eliminate the effects of external vibration acting on a sensor tube.
As a kind of mass flowmeter for measuring a mass flow rate (flux) of a fluid, there is a flowmeter which measures the flux directly by using Coriolis' force which is generated when the fluid flows through a vibrating sensor tube. This kind of mass flowmeter is proposed in a Japanese Laid-Open Patent Application No.54-52570, for example. According to this proposed mass flowmeter, the fluid flows through an approximately U-shaped sensor tube, and the sensor tube is vibrated by a driving force of a vibrator (excitation coil). The Coriolis' force acts in the vibrating direction of the sensor tube, and in mutually opposite directions at the entrance and exit of the sensor tube, so that torsion is introduced in the sensor tube. The torsion angle is proportional to the flux. Hence, a pickup (vibration sensor) for picking up vibration is provided at the torsion positions at the entrance side and the exit side of the sensor tube. The flux is measured by measuring a time difference of output detection signals of the two pickups and measuring the torsion of the sensor tube.
The mass flowmeter described above is provided at an intermediate part of a pipe which normally supplies the fluid, and the inlet side and the outlet side of the mass flowmeter are respectively supported by the pipe. However, a pipe having a small diameter is used for a pipe path in which the flowing fluid has a relatively small flux, and a flexible tube made of a synthetic resin, for example, is used for a pipe path in which the fluid pressure is small. When the pipe path is formed by the pipe having the small diameter or the flexible tube, the mass flowmeter cannot be supported by the pipe or tube and the mass flowmeter must be mounted on the floor or a fixed base such as a bracket which is supported at a predetermined height by a beam.
But if the mass flowmeter is mounted on the fixed base, various external vibrations reach the mass flowmeter via the floor or the fixed base such as the bracket because various kinds of heavy machineries are provided in the factory, for example. Robots, pressing machines, cranes, fork lifts, compressors and machine tools are examples of the heavy machinery. On the other hand, since the proposed mass flowmeter described above is designed to measure the flux by vibrating the sensor tube, an accurate measurement cannot be made if the external vibration reaches the sensor tube or the pickups, and there is a problem in that the measuring accuracy deteriorates when the mass flowmeter is mounted on the floor or the fixed base.
As a measure against the external vibration, it is conceivable to support the mass flowmeter by a vibration absorbing rubber member. In this case, however, it is only possible to absorb the high-frequency vibration, and the low-frequency vibration cannot be absorbed by the vibration absorbing rubber member.